Spatially resolved heat transfer measurements of impinging jets on statistically rough surfaces using an infrared-based gradient sensor

Heat-transfer measurements of jets impinging on rough surfaces are scarce because conventional heated‑thin-foil methods confine high‑resolution measurements to smooth walls. This study presents an infrared‑thermography‑based gradient sensor that enables spatially resolved wall‑heat‑flux measurements on thermally thick plates with embedded, statistically homogeneous roughness. The method is validated against direct numerical simulation and established smooth‑wall datasets for Reynolds numbers in the range $5000 \leq Re \leq 30000$ and nozzle-to-plate distances in the range $2 \leq H/D \leq 5$, showing good accuracy and repeatability. Using two roughness scales ($k_{99}/D = 0.04$ and 0.12), we provide the first systematic dataset of local Nusselt‑number distributions for turbulent impinging jets on rough surfaces at Reynolds numbers relevant for practical applications. Roughness is shown to have little effect at low $Re$, but a pronounced heat‑transfer enhancement appears once a combined threshold in roughness height and Reynolds number is exceeded. For the larger roughness, strong augmentation is already observed for $Re \geq 10000$, especially near $r/D \approx 1$ where wall‑jet shear is highest. ... mehrAt higher Reynolds numbers, roughness appears to fundamentally modify the radial heat‑transfer pattern: the secondary peak, typically observed for smooth‑wall impingement, disappears, giving way to a bell‑shaped Nusselt profile, likely governed by altered near‑wall flow dynamics. These findings offer new insight into heat transfer of rough‑wall jet impingement at high $Re$ and establish a validated measurement technique suitable for future studies integrating heat transfer, flow‑field diagnostics, and wall‑shear measurements.